1. Field of the Invention
This invention relates to scanning tunneling microscope (STM) and more particularly to high resolution imaging of magnetic structures with a non-magnetic tip.
2. Background Art
Imaging magnetic domains and other magnetic structures is important in industry in order to develop improved high density magnetic and magneto-optic storage products. The resolution of a scanning magnetic force microscopy (MFM) is of the order 100 nanometers. The resolution of a tunneling stabilized MFM is also of the order of 100 nanometers. A spin polarized scanning tunneling microscopy (SP-STM) has a much higher resolution than 100 nanometers but requires atomically clean surfaces and ultra high vacuum (UHV) conditions so that the spin polarization of the tunneling electrons is not perturbed by surface contamination. Further, if the spin polarized tip of a SP-STM is magnetic, the stray magnetic fields from the tip can modify the magnetic state of the sample being examined.
In a publication by J. R. Kirtley et al., IBM J. Res. Develop. 32, 414(1988), entitled "Scanning tunneling measurements of potential steps at grain boundaries in the presence of current flow", a scanning tunneling microscope with a Pt-Rh tunneling tip was used at voltages low in comparison to the tunneling barrier potential (typically a few electron volts). A Au-Pd film was examined at room temperature in vacuum wherein the current-voltage characteristic did not pass through the origin, because an externally applied transverse current through the film had changed the potential directly below the tip relative to ground.
A publication by J. P. Pelz et al., Rev. Sci. Instrum. 60, 301 (1989), entitled "Extremely low-noise potentiometry with a scanning tunneling microscope", extremely loss-noise potentiometry was described utilizing an STM to make extremely low-noise potentiometry measurements in metallic materials at room temperature.
In a publication by H. J. Mamin et al., Appl. Phys. Lett. 55, 318 (1989), entitled "Magnetic force microscopy of thin Permalloy films", the magnetic tip of a magnetic force microscope (MFM) was oscillated close to soft magnetic films. Changes in force derivative at the tip result in changes in resonant frequency, which are sensed as changes in the oscillation amplitude. A MFM image was made with the tip was scanned at a constant height of 150 nanometers above the surface of thin-film samples of Permalloy. Images were made clearly showing domain walls and the classic closure structure.
In U.S. Pat. No. 3,846,770 which issued on Nov. 5, 1974, to L. J. Schwee, a polycrystalline thin film strip such as Permalloy is used to store information in a serial manner in the form of reversal domains. The reversal domains are propagated along the hard axis of the thin film strip and then sensed by conventional sensing devices to detect the stored information. A semiconductor element may be placed adjacent to the path of reversal domains. The semiconductor utilizes the Hall effect and a Hall voltage is sensed as a result of the stray magnetic field of the domain as it passes below the semiconductor element.